Sen. Dianne Feinstein of California wants us to know that she is “not a sixth-grader.”

Anyone who saw the recent exchange before the Senate Judiciary Committee between Feinstein and Sen. Ted Cruz of Texas over guns and the Constitution might speculate that the reason she said this is because she couldn’t pass the entrance exam.

As for Cruz, a friend of mine for a decade, it turns out that the most important of the “Senate rules” is unwritten: Thou shalt not embarrass a fellow senator — even one in the opposing party — by making him or her look unprepared, uneducated or uninformed.

Sen. Dianne Feinstein (AP Photo/J. Scott Applewhite)

That’s not always an easy thing to avoid. The rulebook doesn’t say what to do when a Senate colleague who wants to ban certain guns dodges a tough question and then goes on the attack – thus embarrassing herself.

Nevertheless, Cruz, 42, is headed to the principal’s office. His infraction was asking the right question. What Cruz wanted to know was this: Why do liberals cherish the First and Fourth Amendments, but trash the one in between — the Second Amendment?

That’s a brainteaser. Why does the left play favorites with different parts of the Bill of Rights?

Experience teaches that the better the question, the less likely you are to get a straight answer.

That’s what happened here. Feinstein went on the offense. Abandoning reason for emotion, she scolded Cruz for daring to “lecture” her. After all, she said, she had seen the bodies of people killed by gunfire.

She was referring to the assassinations of San Francisco SupervisorHarvey Milk and San Francisco Mayor George Moscone in 1978. Feinstein obviously feels very passionate about limiting people’s access to guns. But who says she gets to decide who gets to own a gun or how many stay on the market?

Enter Cruz. The freshman senator is forcing the Washington establishment – people in both parties – to think. And oh, how they dislike him for it.

Before his election to the Senate, Cruz — who was solicitor general of Texas — built a reputation as one of the best litigators and constitutional lawyers in the country.

What Feinstein, 79, has is a conviction that she – like many liberals – cares more about the victims of gun violence than everyday Americans possibly could.

Ironically, Feinstein told CNN’s Wolf Blitzer that she felt the Texan was “arrogant.”

Look who’s talking. Arrogant is when a person feels so imperial that she doesn’t think that she should have to put up with questions from a colleague.

So what, pray tell, does Feinstein have going for her? Experience. She’s been in the Senate for 20 years. Is this how the game is played? You don’t become an expert in a subject and then go onto a Senate committee. It’s the other way around. Feinstein acknowledged during the exchange with Cruz that, while she feels perfectly comfortable making laws, she herself is “not a lawyer.”

If this were you or me, that fact might deter us from arguing the law and the Constitution with someone who has argued both before the Supreme Court nine times.

Which might lead us to ask the senator from California: “Are you smarter than a sixth-grader?”